. "To evaluate this model, we generated constructs that encode the catalytic domains of human CDC25 or mouse SOS1, either alone (designated cCDC25 and cSOS1, respectively) or terminating in the carboxyl-terminal CAAX membrane-targeting sequence from K-Ras4B (designated cCDC25-CAAX and cSOS1-CAAX, respectively; in CAAX, C is Cys, A is an aliphatic amino acid, and X is Ser or Met)." . . .